The best known technique for near-field optical microscopy is to image through a pinhole that is smaller than the diffraction limited spot size. If the pinhole is placed very close to the object, the pinhole size essentially determines the definition. The fields decay rapidly after passing through the aperture, and for this reason the sample must be brought into close proximity with the pinhole. Near-field optical microscopes based on this principle have been demonstrated by Pohl and others. (See D. W. Pohl, W. Denk and M. Lanz, Appl.Phys.Lett. 44 652 (1984); E. Betzig, M. Isaacson and A. Lewis, Appl.Phys.Lett. 51, 2088-2090 (1987)) In copending application Ser. No. 07/508,224 filed Apr. 12, 1990 assigned to a common assignee, there is described an optical microscope which includes a high refractive index solid immersion lens interposed between the objective lens and the sample being viewed to provide a microscope having improved resolution. By placing the solid immersion lens (SIL) in contact with or very close to the sample, it can be used for imaging in two modes: the near field mode and the internal imaging mode. These two modes are illustrated in FIGS. 1 and 2 which show a solid immersion lens 11 interposed between an objective lens 12 and a sample 13.
In its near-field mode of operation, illustrated in FIG. 1, the microscope uses both the evanescent fields just outside the flat surface of the SIL and fields that propagate in air to image objects placed close to the SIL. The propagating fields allow the system to be easily focused and the evanescent fields improve the resolution in air by a factor 1/n where n is the index of refraction of the solid immersion lens material. Thus, near-field operation of this kind makes it possible to partially circumvent diffraction effects which limit the transverse resolution of scanning confocal and standard optical microscopes to approximately a half wavelength in air.
In the internal imaging mode, an SIL is used which has the same refractive index as the medium being examined, and the SIL is placed in contact with or very close to the sample (FIG. 2). In this case, the beam may be focused into the interior of the sample without aberration, and a definition which depends on the wavelength inside the sample rather than that in air.
In optical lithography, there is a continuing need for improved definition in the transfer of patterns from a photomask to a photoresist used in the processing of semi-conductor devices such as integrated circuits. It is predicted that integrated circuits having line widths below 0.5 .mu.m will be in volume production by the mid-nineteen nineties.